Abstract
The plant-pathogenic bacterium Xylella fastidiosa was first reported in Europe in 2013, in the province of Lecce, Italy, where extensive areas were affected by the olive quick decline syndrome, caused by the subsp. pauca. In Alicante, Spain, almond leaf scorch, caused by X. fastidiosa subsp. multiplex, was detected in 2017. The effects of climatic and spatial factors on the geographic distribution of X. fastidiosa in these two infested regions in Europe were studied. The presence/absence data of X. fastidiosa in the official surveys were analyzed using Bayesian hierarchical models through the integrated nested Laplace approximation (INLA) methodology. Climatic covariates were obtained from the WorldClim v.2 database. A categorical variable was also included according to Purcell’s minimum winter temperature thresholds for the risk of occurrence of Pierce’s disease of grapevine, caused by X. fastidiosa subsp. fastidiosa. In Alicante, data were presented aggregated on a 1 km grid (lattice data), where the spatial effect was included in the model through a conditional autoregressive structure. In Lecce, data were observed at continuous locations occurring within a defined spatial domain (geostatistical data). Therefore, the spatial effect was included via the stochastic partial differential equation approach. In Alicante, the pathogen was detected in all four of Purcell’s categories, illustrating the environmental plasticity of the subsp. multiplex. Here, none of the climatic covariates were retained in the selected model. Only two of Purcell’s categories were represented in Lecce. The mean diurnal range (bio2) and the mean temperature of the wettest quarter (bio8) were retained in the selected model, with a negative relationship with the presence of the pathogen. However, this may be due to the heterogeneous sampling distribution having a confounding effect with the climatic covariates. In both regions, the spatial structure had a strong influence on the models, but not the climatic covariates. Therefore, pathogen distribution was largely defined by the spatial relationship between geographic locations. This substantial contribution of the spatial effect in the models might indicate that the current extent of X. fastidiosa in the study regions had arisen from a single focus or from several foci, which have been coalesced.
Highlights
Xylella fastidiosa (Wells et al, 1987) is a gram-negative plantpathogenic bacterium of the Xanthomonadaceae family
All sampled plant species were included in the analysis, as they were considered susceptible to X. fastidiosa (EFSA, 2020)
Non-sampled grid cells were removed, since most of them corresponded to mountain peaks with difficult access and/or the absence of host plants; the study area had an extension of 638 km2
Summary
Xylella fastidiosa (Wells et al, 1987) is a gram-negative plantpathogenic bacterium of the Xanthomonadaceae family It colonizes the xylem tissues of a wide range of plant species: The latest update of the host plants database includes 595 species belonging to 85 different families (EFSA, 2020). It is only in some combinations of host plant and bacterial strain that infections can result in some of the most destructive diseases, such as Pierce’s disease (PD) of grapevine and olive quick decline syndrome (OQDS), threatening several crops of great economic importance. Fastidiosa causes, among others, PD and almond leaf scorch; X. fastidiosa subsp. Multiplex has been found in about 140 plant species and is associated with leaf scorch diseases in numerous tree species, including almond (EFSA, 2020). The X. fastidiosa strain isolated from Pyrus pyrifolia in Taiwan has been proposed to form a distinct species in the genus Xylella: the X. taiwanensis sp. nov (Su et al, 2016)
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